1. Field of the Invention
The present invention relates to a mapping method for frequency and orthogonal frequency division multiplexing (OFDM) symbol regions of a signal transmitted on downlink in a cellular OFDM wireless packet communication system.
2. Discussion of the Related Art
When transmitting/receiving a packet in a mobile communication system, a receiver should inform a transmitter as to whether or not the packet has been successfully received. If the reception of the packet is successful, the receiver transmits an acknowledgement (ACK) signal to cause the transmitter to transmit a new packet. If the reception of the packet fails, the receiver transmits a negative acknowledgement (NACK) signal to cause the transmitter to re-transmit the packet. Such a process is called automatic repeat request (ARQ). Meanwhile, hybrid ARQ (HARQ), which is a combination of the ARQ operation and a channel coding scheme, has been proposed. HARQ lowers an error rate by combining a re-transmitted packet with a previously received packet and improves overall system efficiency. In order to increase throughput of the system, HARQ demands a rapid ACK/NACK response from the receiver compared with a conventional ARQ operation. Therefore, the ACK/NACK response in HARQ is transmitted by a physical channel signaling method. The HARQ scheme may be broadly classified into chase combining (CC) and incremental redundancy (IR). The CC method serves to re-transmit a packet using the same modulation method and the same coding rate as those used when transmitting a previous packet. The IR method serves to re-transmit a packet using a different modulation method and a different coding rate from those used when transmitting a previous packet. In this case, the receiver can raise system performance through coding diversity.
In a multi-carrier cellular mobile communication system, mobile stations belonging to one or a plurality of cells transmit an uplink data packet to a base station. That is, since a plurality of mobile stations within one sub-frame can transmit an uplink data packet, the base station must be able to transmit ACK/NACK signals to a plurality of mobile stations within one sub-frame. If the base station multiplexes a plurality of ACK/NACK signals transmitted to the mobile stations within one sub-frame using CDMA scheme within a partial time-frequency region of a downlink transmission band of the multi-carrier system, ACK/NACK signals with respect to other mobile stations are discriminated by an orthogonal code or a quasi-orthogonal code multiplied through a time-frequency region. If quadrature phase shift keying (QPSK) transmission is performed, the ACK/NACK signals may be discriminated by different orthogonal phase components.
When transmitting the ACK/NACK signals using CDMA multiplexing scheme in order to transmit a plurality of ACK/NACK signals within one sub-frame, a downlink wireless channel response characteristic should not be greatly varied in a time-frequency region in which the ACK/NACK signals are transmitted. This is because if orthogonality is maintained between the multiplexed different ACK/NACK signals, a receiver can obtain satisfactory reception performance without applying a special receiving algorithm such as channel equalization. Accordingly, the CDMA multiplexing of the ACK/NACK signals should be performed within the time-frequency region in which a wireless channel response is not significantly varied. However, if the wireless channel quality of a specific mobile station is poor in the time-frequency region in which the ACK/NACK signals are transmitted, the ACK/NACK reception performance of the mobile station may also be greatly lowered.
Accordingly, the ACK/NACK signals transmitted to any mobile station within one sub-frame may be repeatedly transmitted over separate time-frequency regions in a plurality of time-frequency axes, and the ACK/NACK signals may be multiplexed with ACK/NACK signals transmitted to other mobile stations by CDMA in each time-frequency region. Therefore, the receiver can obtain a time-frequency diversity gain when receiving the ACK/NACK signals.
However, in a conventional physical hybrid ARQ indicator channel (PHICH) mapping method, there exists a defect that PHICH groups between neighbor cells have difficulty avoiding collision as illustrated in FIG. 1.